When magneto-resistive (MR) sensors are used in an open-loop mode for magnetic field sensing, the field value is derived directly from the MR sensor's resistance. This requires an accurate resistance reference point and accurate estimation of the sensor sensitivity to the external field. MR sensors, especially tunneling magneto-resistance (TMR) sensors, can show large resistance changes after exposure to long term stresses that are significant enough to affect the sensing accuracy. However, when sensor resistance is normalized relative to the max, min, or mean values of each sensor's transfer curve, the resulting population shifts and inter-sensor variations can be fully compensated for. Long-term accurate open-loop operation is therefore possible if normalized values of this sort are used.
When used for magnetic field sensing, MR sensors have the advantage of much higher sensitivity when compared to the conventional Hall sensors that are being used in today's commercial products. State-of-the-art magnetic tunnel junction (MTJ) sensors can be an order of magnitude more sensitive than Hall sensors. This suggests the possibility of employing magnetic field sensing in applications that were originally not achievable by Hall sensors. For example, MR sensors [1-2] have been used to sense the magnetic field generated by an electric current i.e. as current sensors.
FIG. 1 shows an open-loop configuration of the prior art wherein a current source powers MR sensor 9 and the voltage across the sensor is compared to reference voltage Vref, which (preferably) is the same as the sensor 9 voltage when there is no external magnetic field present. Thus, output voltage Vout is proportional to the resistance change of the sensor 9. If the sensor 9 resistance has a reasonably linear response to the external field, Vout will then be a measure of the magnetic field at sensor 9. Circuits such as that shown in FIG. 1 are limited by the fact that they use the resistance of the MR sensor directly. Open-loop structures, such as FIG. 1, are sensitive to absolute resistance fluctuations in which the resistance shifts during operation, for example through thermal, electrical and mechanical stresses. These will directly affect the accuracy of the measurement. Furthermore, even though, in some cases, the prior art has devised ways to deal with short-term sensitivity variations of this sort, long-term resistance shifts over time are considered to be an even more serious drawback for the FIG. 1 open-loop scheme.
FIG. 2 illustrates how the zero field resistance, R0, of two (randomly selected) TMR sensors can change over time during which they were in power-on mode and thereby exposed to a stress temperature significantly greater than room temperature. Seen there is an average resistance increase of ˜4.5% over the initial zero field resistance values as well as a difference of ˜1% between the resistance increases of the two devices. This data indicates that, if changes of this type go uncorrected, open-loop schemes such as the one seen in FIG. 1 will certainly suffer accuracy degradation over time.